1. Field of the Invention
The present invention is directed to liquid crystal display devices, and in particular, to color liquid crystal display devices having a dead front appearance, and to methods of making such liquid crystal displays. These displays are particularly useful when incorporated into control panels of larger machines such as, for example, copiers.
2. Description of Related Art
Liquid crystal devices are, in general, well known, and are frequently incorporated into control panels of larger machines (e.g., automobiles or automated office equipment) for conveying information regarding the status of the machine, or the location of components of the machine to an operator.
For example, the Xerox 4050 printer includes a control panel having a liquid crystal display (LCD) assembly that provides an illustration of the printer and its components, and an alphanumeric vacuum fluorescent display (VFD) that provides an operator with instructions or other information. The VFD and LCD assembly can be simultaneously controlled so that as the VFD provides an instruction involving the manipulation of some machine component (e.g., "fill tray with paper" or "door is open"), that component (the tray or the door) is displayed on the LCD assembly.
U.S. Pat. No. 4,475,806 to Daughton et al discloses a copier display panel including a VFD and LCD assembly which are simultaneously controlled to permit an operator to interact with the copier. Accordingly, the disclosure of U.S. Pat. No. 4,475,806 is incorporated herein by reference.
FIG. 1 is an exploded view of the VFD and LCD assembly on the control panel of the 4050 printer. The VFD 110 and the LCD assembly 120 are mounted on a common mounting board 100. The LCD assembly 120 includes a lamp reflector and support housing 122 which is attached directly to the mounting board 100 with, for example, snap features. A fluorescent lamp 124 attached to a lamp housing/reflector 126 is mounted to the display console front bezel (not shown), and acts as the light source for the LCD assembly. A ground glass light diffuser 130 is placed in the lamp reflector and support housing 122 to provide a uniform amount of light to the entire LCD. A liquid crystal display glass 132 (described in more detail below) which includes a liquid crystal material sandwiched between two electrode-containing glass plates, a pair of polarizers, and a plurality of luminance balanced color patches encircled by a gray surround formed on one of the glass plates is placed over the light diffuser 130. The electrodes on the glass plates of display glass 132 are sized and shaped to define image elements, and include lead lines which provide electrical contact to a tape cable 128 by zebra strips 129. The tape cable is then connected to a controller for actuating the LCD electrodes. A machine outline overlay 134 is placed over display glass 132. Overlay 134 is a transparent mylar sheet having a white outline of the copier machine stenciled thereon. Front frame 136 is placed on overlay 134 and attached to support housing 122 by screws. Accordingly, light diffuser 130, LCD glass 132, zebra strips 129, tape cable 128, and overlay 134 are sandwiched between, and held in place by, front frame 136 and support housing 122.
Since the electrodes on the glass plates are the same size and shape as the image element that they define, they are known as "mimic"type electrodes. That is, they mimic the shape of their image element (e.g., a copier door, or a paper tray, etc.). A second type of electrode used in LCDs is the "bit" type. Bit type electrodes are small dots (or squares), each of which only defines a small part of an image element. Bit type electrodes are usually provided in matrices, in which a large number of bit electrodes must be activated at one time to light a large image element. Bit electrodes have the advantage of allowing the information in the display to be changed. However, bit electrode displays must usually be multiplexed. The electrical signals associated with multiplexing result in lower contrast ratios, restricted viewing angles, and longer response times. For applications such as office machines where the viewer is not in a fixed position, and in particular where a dead front is required, high contrast ratio and wide viewing angle are required. These requirements are best met by displays with mimic electrodes driven by individual electrical signals to each element. These signals allow the contrast ratio, viewing angle, and response time to be maximized.
The 4050 LCD assembly, and in particular the LCD glass 132, is constructed generally according to the teachings of U.S. Pat. Nos. 4,506,956 and 4,527,864 to Dir. The Dir patents disclose a multicolor display including a high contrast liquid crystal device, a light source, and a luminance balanced color mask. The liquid crystal device is the transmissive, twisted-nematic type having high extinction ratio polarizers on the input and output sides, and functions as a light shutter for the display. The liquid crystal material has a dichroic dye admixed therein to prevent buildup of contrast limiting orthogonal mode electric vector of polarized light. The color mask contains different colored patches with gray surround about them. Each colored patch is luminance balanced with each other and the gray surround in order that each different colored data presented are of substantial equal brightness. For a more detailed description of the problems addressed by the addition of dichroic dyes to liquid crystal materials, see U.S. Pat. Nos. 4,506,956 and 4,527,864 to Dir, the disclosures of which are incorporated herein by reference.
In the Dir patents, dichroic dyes are mixed with the liquid crystal material so as to absorb an orthogonal component of the light which would normally pass through the liquid crystal light shutter when in the off-state. (Light also leaks through the areas surrounding each light shutter (between the electrodes) where the liquid crystal material is permanently in the off-state.) Theoretically, the light shutters of Dir should have a high contrast ratio and produce a uniformly dark state or dead front when switched off. However, light continued to leak through the display of Dir even when in the off-state. Dichroic dyes do not perfectly absorb only a single polarization state of the transmitted light due to (1the limited dichroic ratio of the dye itself, and (2) imperfect alignment of the dye in the liquid crystal host (order parameter). Because of this, the shutters, when switched on, have a lower transmittance than desired, requiring a high level of illumination. In addition, the optimum absorbance of the orthogonal component of light is obtained only at certain values of the thickness of the liquid crystal layer, depending on the birefringence of the dye/liquid crystal mixture, causing high tolerance requirements for constructing such a liquid crystal display with a uniform dead front (i.e., differences in the thickness of the liquid crystal material layer cause differences in the light transmission characteristic thereof). Additionally, the glass spacer rods inserted between the electrode containing glass plates of the light shutter provide a pathway for the transmission of light through the light shutter when in the off-state. Other contributors to bleed-through include defects in the liquid crystal alignment layer and errors in the rotational angle of the liquid crystal alignment layers and polarizers.
Accordingly, in order to de-emphasize (i.e., to make less visually noticeable) the leakage of light through the light shutters (and associated color patches) of the display, Dir provides a gray background surrounding the color patches, and luminance balances the color patches and gray surround. The luminance balancing makes it more difficult for a person to visualize the light bleeding through the color patches when in the off-state. Thus, the Dir display does not provide a true dead front since light does pass therethrough when in the off-state. Instead, Dir makes use of the threshold for detection of contrast of the human eye so that light bleeding through the color patches in the off-state is theoretically undetectable against the gray surround (through which some light also leaks).
While the 4050 LCD works fairly well in bright ambient lighting conditions, when the amount of ambient light is low, light bleeding through the color patches is noticeable by an operator. This leads to confusion since operators frequently believe that a component of the copier must be adjusted or serviced (since that component appears to be lighted in the LCD), when, in fact, no adjusting or servicing is required. Additionally, operators sometimes cannot determine which components require servicing because multiple components on the LCD appear to be lighted.
The luminance balancing technique for providing an apparent dead-front display also is not satisfactory when textual information is conveyed by the color liquid crystal light shutters. The nature of text requires that it be high contrast (against a background; such as black against white) rather than luminance balanced. That is, colored-text (including white) is more easily readable when displayed against a dark background than when displayed against a partially lighted gray background. Furthermore, the spatial frequencies of text make it easily noticeable unless no light passes therethrough. Accordingly, the LCD in the 4050 printers does not include textual data.
A number of patents disclose LCD assemblies having two or more light shutters in series.
U.S. Pat. No. 4,917,465 to Conner et al discloses a color display system wherein three supertwisted nematic birefringent LCD panels are stacked together and tuned to a different primary color component (yellow, cyan or magenta). Several polarizers are interspersed within the stack of panels and at each end of the system. The polarizers may be colored in order to improve color and brightness. A "black" LCD in combination with the color LCDs is disclosed for improving the contrast ratio.
U.S. Pat. No. 4,547,043 to Penz discloses a stacked LCD graphics display wherein a series of LCDs are used to increase contrast ratio. Each of the LCDs receive identical driving signals. Each LCD modulates the incoming light by the same percentage.
U.S. Pat. No. 4,637,687 to Haim et al discloses a cascaded, dual cell transflective LCD wherein the two LCD cells improve the contrast ratio of the LCD in a transmissive and a reflective state. A transreflective element is placed between the cells. Additionally, the cell display segments (electrodes) of the rear cell can be made larger than the segments in the front cell.
U.S. Pat. No 4,927,240 to Stolov et al discloses a multiple LCD wherein at least two twisted nematic LCDs are used to improve contrast ratio. A three cell LCD with two polarizers can theoretically have a contrast ratio of 8000:1. The use of a color or multicolor layer between each LCD is also disclosed.
U.S. Pat. No. 4,929,061 to Tominaga et al discloses a color LCD device wherein a liquid crystal panel has a plurality of primary color pixel components and a liquid crystal shutter associated with each primary color component. In addition, the device includes a back light unit which is placed behind the LCD. The panel, the shutters and the back light unit are all part of the same device.
U.S. Pat. No. 4,595,259 to Perregaux discloses a liquid crystal image bar for marking a photoreceptor. A light shield of an opaque metal or organic film is deposited on the inner face of one of the glass LCD substrates to block stray light from passing therethrough.
U.S. Pat. No. 4,834,506 to Demke et al discloses an LCD wherein inter-electrode spaces are made dark by printing a matrix of black lines wherever electrode material is removed from a glass substrate.
U.S. Pat. No. 4,896,945 to Ooba et al discloses a liquid crystal cell display wherein light shielding layers are formed between adjacent transparent electrodes.
The disclosures of the above-referenced patents are incorporated herein by reference.